The invention relates to a method of curing a film on a surface of a cathode ray tube.
Cathode ray tubes are used, inter alia, in television receivers, computer monitors and oscilloscopes.
A method of the type mentioned in the opening paragraph is known per se. A film is applied to a surface of the display window of a cathode ray tube. This film is sprayed in the form of a liquid on the surface, whereafter it is dried and cured. Customarily, the film is cured in a furnace. The high temperature in the furnace causes curing of the film. The curing time of the film ranges of from 30 minutes to 1 hour. Said method is less suitable for mass production. In a production line the aim is to adjust the duration of the various process steps to each other. A display window or display tube can then be subjected to the various process steps either sequentially or "in-line". A process step typically takes approximately 1 minute. If the duration of a process step substantially exceeds the average duration, then said process step cannot be carried out "in-line", as the velocity in the production line is generally determined by the slowest process step. The relevant process step must be carried out outside the production line. To this end, the display window or display tube must be removed from the production line and an arrangement for carrying out said process step must be set up alongside the production line. This results in an increase of the production costs as well as of the risk of breakage.
It is an object of the invention to provide a method of the type mentioned in the opening paragraph, which is more suitable for "in-line" application.
To this end, a method in accordance with the invention is characterized in that the film is cured by a laser beam which is incident on a limited area of the film and which scans the surface.
The invention is based on the insight that by means of a laser beam the part of the film situated in said area can be cured within a few seconds. It has been found that the film can be cured in a period of time ranging from ten seconds to several minutes by scanning the surface with said laser beam. The movement of the laser beam over the surface has the advantage that the temperature of the element itself, for example the display window, increases only slightly. Only the film and the uppermost layer of the surface (i.e. the portion of the element situated directly underneath the film) are subject to a substantial increase in temperature. By virtue thereof, thermal stresses occur hardly, or not at all. Consequently, breakage caused by thermal stresses is precluded. The latter advantage is particularly important in embodiments of the method in accordance with the invention which are characterized in that the film is applied to the outside of an evacuated cathode tube, for example the display window or the cone of the evacuated cathode ray tube. In general, an evacuated display tube cannot be exposed to a high temperature for a prolonged period of time. In the known method, the temperature of the display tube and hence of the furnace must be limited to approximately 100.degree. C.-200.degree. C. At higher temperatures, there is a substantial risk of breakage of the display tube. In the method in accordance with the invention, the temperature of the part of the film which is irradiated by the laser beam is several hundred degrees centigrade. The temperature of the display tube as a whole, however, is much lower. The latter advantage (reduced risk of breakage) is important, in particular, for display tubes comprising a display window having a diagonal whose length is in excess of 50 cm. The time which is necessary to heat the tubes to the desired temperature in a furnace increases as the tubes are larger, and hence the risk of breakage increases also. A preferred embodiment of the method in accordance with the invention is characterized in that the film absorbs little laser light and the release of energy is concentrated in the area underneath the film to a depth of less than 10 .mu.m.